The present invention relates to a process for controlling antennas as well as a corresponding device adapted particularly for a free hand system of an automotive vehicle.
A free hand system permits access to a vehicle and starting the latter without having to use a mechanical key. The user of the vehicle is thus simply provided with an electronic card (also called hereinafter a badge) which is detected and recognized by a control and management device associated with the antennas disposed onboard the vehicle. If the card is identified by the control and management device as being a card authorized for the vehicle, the bearer of this card can enter the vehicle by simply grasping a door handle and starting the motor of the vehicle by simple action on a button.
Generally, for such a free hand system, a group of antennas is adapted to detect the presence of the badge outside the vehicle and another within the latter. If an authorized badge is detected outside the vehicle, the control and management system will permit access to the interior of the vehicle without however permitting starting, whilst if this badge is within the vehicle, starting the motor can take place.
To locate the badge, a signal is sent to a group of antennas which thus carries out a corresponding emission whilst no signal is sent to the other antennas such that they remain inactive. The antennas used are LF antennas (Low Frequency) with a small field, thereby permitting carrying out good location of the corresponding badge.
The control and management device includes electronics comprising particularly amplifier circuits to send high power signals to the antennas. This control electronics of the antennas does not permit low cost sending of a high power signal to a group of antennas without sending a corresponding weak signal to the antennas adapted to remain inactive. This is due to a problem of diaphony, known to those skilled in the art, between the different amplifier circuits.
In a free hand system for an automotive vehicle, this problem of diaphony can lead to errors of location of the badge of the vehicle. Thus for example, when the internal antennas emit a high powered signal in the direction of the badge in order to detect it, the external antennas themselves also emit a signal, albeit of low power, but which can be detected by the badge. If the badge is thus located immediately adjacent an external antenna, it will be identified and located as being within the vehicle. In the case in which the driver is outside the vehicle, with the badge in his rear pocket or his trousers against the handle of the door which generally includes the antenna, the badge will be considered as being inside the vehicle and during an attempt to start up, for example by an infant playing within the vehicle, the motor will be activated.
The solution of the prior art to resolve these problems of diaphony is the total disconnection of the antenna that should remain inactive, from the rest of the circuit, by for example a relay. This solution is effective but of high cost.
The present invention thus has for its object to provide a low cost solution to the problem of diaphony described above.
To this end, it proposes a process for controlling at least two groups of antennas according to which a modulated signal is emitted by a first group of antennas whilst the antennas of a second group emit a parasitic modulated signal that is comparable but of lesser amplitude.
According to the invention, a signal of unmodulated amplitude comprised between the amplitude of the parasitic signal and the amplitude of the signal emitted by the first group of antennas, is superposed on the parasitic signal emitted by the antennas of the second group.
Thus, rather than seeking to improve the performance of the electronics to suppress the parasitic signal on the antennas which it is desired to render inactive, an unmodulated signal is emitted by these antennas. The signal thus emitted is the superposition of the parasitic signal and the unmodulated signal. The amplitude of the unmodulated signal is selected such that the interpretation of the overall signal emitted by the antennas that should initially remain inactive, will not be possible.
Preferably, the amplitude of the unmodulated signal permits just masking the parasitic signal. It is thus for example comprised between twice the amplitude of the parasitic signal and half the amplitude of the signal emitted by the first group of antennas. This amplitude should not be too great, because the field thus emitted by the external antennas (with the signal that cannot be interpreted by the badge) will substantially overflow the interior of the vehicle and thus mask the field emitted by the internal antennas (which is itself modulated to be comprehended by the badge).
In one embodiment, the signal emitted by each antennas can be the superposition of a modulated signal, parasitic or not, and an unmodulated carrier.
The present invention also relates to a control device for antennas distributed in at least two groups of antennas comprising a modulator, a multiplexer disposed downstream of the modulator as well as an amplifier circuit disposed between each group of antennas and the multiplexer. According to the invention, a branch circuit comprising a resistance connects a point upstream of the modulator to a point downstream of the multiplexer, upstream of an amplifier circuit corresponding to a group of antennas. In this way, an unmodulated signal is superposed on the signal of a group of antennas.
In a modified embodiment, there corresponds to each group of antennas a branch circuit comprising a resistance.
The present invention finally relates to a system of free hand access to a vehicle comprising external antennas and internal antennas, an electronic identification card, as well as control electronics, characterized in that it comprises an antenna control device such as described above.